Conventionally, the production of elastomeric compounds used in the manufacturing of tyres is performed batchwise by means of batch mixing devices, e.g. internal mixers such as, for example, Banbury® mixers, having two counter-rotating rotors which exert an intensive mixing action to masticate the elastomeric polymer(s) and to incorporate and thoroughly disperse therein the other components usually present in the elastomeric compounds such as, for example, reinforcing fillers, lubricating aids, curatives and other additives.
The production of elastomeric compounds using batch mixing devices shows many drawbacks, particularly a poor heat dissipation and thus a scarce temperature control, mainly due to an unfavorable ratio between material volume and mixer surface area. To improve dispersion in the elastomeric polymer(s), the various components, and particularly the reinforcing fillers, are usually incorporated into the elastomeric polymer(s) in batches distributed in a plurality of mixing operations separated by cooling and storage steps. Usually, the temperature sensitive components, such as crosslinking agents and accelerators, are added only during the final mixing step, after the cooling of the elastomeric compounds below a predetermined temperature (usually below 110° C.) to avoid premature crosslinking (“scorching” phenomena).
Therefore, the production of elastomeric compounds using batch mixing devices, although still remaining the most widely used production process in the tyre industry, is time and energy consuming and does not guarantee an effective control on the properties of the resulting elastomeric compounds, particularly with regards to dispersion homogeneity of reinforcing fillers into the elastomeric polymer(s). Variation in the added amounts of individual components, timing of addition and discharge from the mixers, initial temperature of the raw materials, and fluctuations of shear forces inside the material during mixing, all contribute to batch-to-batch variation.
To overcome the limitations of the batchwise processes above disclosed, many attempts have been made by the rubber industry for setting up production processes based on extrusion techniques analogous to those commonly employed in the processing of thermoplastic polymer materials.
For example, U.S. Pat. No. 4,897,236 discloses a process and an apparatus for continuously producing a rubber mixture, wherein the ingredients of the mixture are fed, masticated and homogenized in a twin-screw extruder. The resulting mixture is divided into a first and a second portion. The first portion is discharged, while the second portion is recycled for further homogenization and for mixing with fresh batches of the ingredients being fed to the extruder. The recycled portion is circulated to and returned from a cooled, annular chamber exterior to the extruder chamber, said annular chamber having outflow and inflow passages communicating with the interior of the extruder.
U.S. Pat. No. 5,626,420 discloses a continuous mixing process and apparatus, wherein base elastomer(s) and other components are continuously dosed and introduced into a mixing chamber formed of a stator and a rotor rotating therein, preferably a single screw extruder. The introduced components advance within the mixing chamber along zones of propulsion and mixing. To improve dispersion and homogenization of the rubber components, the filling rate of the mixing chamber in at least certain mixing zones is lower than 1. To properly introduce the components, and particularly the rubber base, into the mixing chamber, force feeding means are used, such as volumetric pumps (e.g. gear pumps).
United States Patent Application US 2004/0094862 discloses a multi-shaft extruder with at least two shafts for compounding and/or molding an elastomer staggered with filler, in particular rubber, with at least softener and/or additives. The extruder comprises the following in succession in the direction of product transport: a feed zone, into which the elastomer and softener and/or additives are metered; a mastication/plasticization zone with at least one kneading element, into which the elastomer with the softener and/or additives is transferred to a flowable, cohesive mixture; a dispersing zone with at least one additional kneading element, in which the filler in the elastomer is comminuted and distributed; and the kneading elements having a comb and the extruder having a casing inner wall, and wherein a gap with a gap width Z of about 1/100 to about 1/10 of the kneading element diameter D is present between the comb of the kneading elements and the casing inner wall of the extruder. In one embodiment, the multi-shaft extruder is a ring extruder.
PCT Patent Application no. WO 03/009989 discloses a process and an apparatus for the continuous production of elastomeric compositions by means of at least one extruder, wherein the minor ingredients are used in the form of subdivided products. The products include at least one of the minor ingredients dispersed in a thermoplastic binding agent and are conveyed to a dosing device by means of a pneumatic conveying line, to be accurately metered and continuously fed into the extruder.